This invention relates to communication terminal equipment to be connected to an integrated services digital network. More particularly, this invention relates to communication terminal equipment that permits two-way communications to be performed over a complex communications network having an integrated services digital network interconnected with existing multiple communication networks and that has a capability for calling again a receiver terminal of interest when a communication trouble has occurred.
Facsimile and other types of communication terminal equipment have chiefly been connected to telephone networks and packet-switched public data networks (hereinafter abbreviated as "PSPDN"). In recent years, such communication terminal equipment is increasingly connected to an integrated services digital network which is adapted to handle digital information (which network is hereinafter abbreviated as "ISDN").
The ISDN concept refers to a communications network in which various kinds of communication machines such as telephones, facsimile equipment and telexes that are connected to data service units (DSU) through buses are designed to handle information in a standardized digital format, with all communication services to those communication machines for a particular DSU being implemented with the same number.
Various communication troubles can occur in telephone networks, PSPDNs and ISDNs as exemplified by the failure to connect a source terminal and a receiver terminal, as well as a disconnected line that occurs during communications. If a certain communication trouble occurs, disconnect information as appropriate for the situation under which the trouble has occurred is sent back to the source terminal. Each terminal equipment is provided with a repeated call management table which is descriptive of the unique correlation between disconnect information and the condition for making a repeated call. Upon receiving a certain type of disconnect information, the terminal equipment refers to that table on the basis of the received disconnect information and makes a repeated attempt to call the receiver terminal under the condition that is appropriate for that disconnect information.
Since the contents of disconnect information differ from one communications network to another, the repeated call management table in each terminal equipment also depends on which communications network it is connected to. For example, the terminal equipment connected to a PSPDN has a repeated call management table a, the terminal equipment connect to a telephone network has a repeated call management table b, and the terminal equipment connected to an ISDN has a repeated call management table c. Thus, each terminal equipment has had a single kind of repeated call management table in accordance with the communications network to which it is connected.
In the early stage of ISDN introduction, an important objective is to establish efficient interconnection between the ISDN and other existing networks. Particularly crucial is the interconnection with telephone networks and PSPDNs which are two most common communication networks.
FIG. 2 shows the basic layout for Case A which has been specified by the CCITT as regards the interconnection between ISDNs and PSPDNs. The term "Case A" as used herein means one from of services to a PSPDN that is connected to an ISDN. As shown in FIG. 2, the ISDN does not have a packet switching capability in Case A and functions only as a physical subscriber loop between a packet terminal 20 connected to the ISDN and a packet switching unit in the PSPDN. If the ISDN packet terminal 20 requests a packet-mode communication via a D-channel, the ISDN switching unit sets up a 64 kb/s unrestricted digital path between the packet terminal 20 and an ISDN access unit board (AU). As a result, a subscriber loop is completed between the ISDN packet terminal 20 and the PSPDN and, thereafter, call control from the packet terminal 20 will be performed by a procedure that is entirely the same as in the call control from the terminal directly accommodated in the PSPDN.
In the above-described configuration of Case A, the communications network to be handled is the ISDN until the packet terminal 20 is connected to the AU, so network control is performed in accordance with the procedure of ISDN call control. However, when a communication line is set up between the packet terminal 20 and the AU, the communications network to be handled is switched to the PSPDN and subsequent call control is performed in accordance with the procedure specified by CCITT's Recommendation X.25. Therefore, it a communication trouble occurs in the ISDN procedure which is followed until the packet terminal is connected to the AU, the disconnect information specified by the ISDN call control procedure is sent back and, if a communication trouble occurs in the procedure of Recommendation X.25, the disconnect information specified by the X.25 call control procedure is sent back. In the former case, the repeated call management table c must be referenced and in the latter case, the repeated call management table a must be referenced.
In fact, however, the communication terminal 20 has only the repeated call management table c and is unable to deal with the second type of disconnect information in a satisfactory manner. In short, each of the communication terminals used in the prior art communications system is equipped with only a single kind of repeated call management table, so if different types of disconnect information are sent back for different communication networks, repeated calls cannot be controlled in normal way.